The present invention relates to a control system for electrochromic mirrors for use, for example, in automobiles and more particularly to a control system for an inside electrochromic (IEC) mirror and one or more outside electrochromic (OEC) mirrors, which are controlled by a glare signal generated within the vehicle.
Various electrochromic mirror and electrochromic window systems (hereinafter xe2x80x9celectrochromic elementsxe2x80x9d) are generally known in the art. Such systems normally include a plurality of electrochromic elements. For example, in automotive applications, electrochromic elements are known to be used for both the rearview mirror and one or more sideview mirrors as well as in window applications for sun load control. It is known that the reflectance of electrochromic elements used as mirrors (or transmittance in the case of electrochromic elements used for window applications) is a function of the voltage applied to the electrochromic element, for example, as generally described in commonly assigned U.S. Pat. No. 4,902,108, which is hereby incorporated by reference. Because of this characteristic, such electrochromic elements are known to be used in systems which automatically control glare from external light sources in various automotive and other applications. In automotive applications, the 12-volt vehicle battery is used as the electrical power source for the electrochromic elements. The electrochromic elements generally operate at a nominal voltage of about 1.2 volts. Since the actual electrochromic element voltages are relatively low compared to the supply voltage, it is known to use a single drive circuit for multiple electrochromic elements. In such applications, the electrochromic elements for the inside and outside mirrors are known to be connected either in series, parallel, or series parallel and driven from a single drive circuit.
In order to prevent damage to the electrochromic elements as well as control their reflectance, the voltage across each electrochromic element must be rather precisely controlled. However, it is known that the resistance of the electrochromic elements may vary as a function of temperature. Thus, in applications with the electrochromic elements being used both inside and outside the vehicle, the temperature difference between the inside and outside electrochromic elements can be relatively significant which can make relatively precise control of the electrochromic elements difficult.
There are other factors which make relatively precise control of the electrochromic elements difficult. For example, in known systems, a glare signal, typically generated within the vehicle, is transmitted by hardwiring to the OEC elements used for the sideview mirrors. The glare signal is used to control the reflectance of the electrochromic elements used for the sideview mirrors. As mentioned above, the OEC elements are normally connected in either series, series parallel, or in parallel with the IEC element used for the rearview mirror assemblies often requiring the voltage to the OEC elements to be scaled or offset. It is known that electrochromic elements typically require a low voltage drive, typically 1.2-1.4 volts to achieve minimum reflectance. As such, a drive voltage accuracy of 0.1 volts or better is required to maintain adequate glare control. Unfortunately, the ground system in an automotive environment can have differences in ground potential exceeding 2.0 volts under some conditions, which can drastically affect the operation of the electrochromic elements. In order to resolve this problem in known automotive applications OEC elements, relatively heavy gauge conductors are typically routed to each of the OEC elements transmission of the glare signal, which increase the cost and weight of installing such a system in an automobile.
There are other problems associated with the relatively accurate control of OEC elements. In particular, OEC elements can be classified according to three major types: flat, convex, and aspheric. The effective magnification or reflectance levels differ for each of the different curvature types. For example, flat mirrors are known to have the highest effective reflectance or magnification (i.e., 1 to 1) while the aspheric and convex mirrors provide relatively lower reflectance (i.e., 1 to 3 and 1 to 4, respectively) depending upon the degree of curvature. The different reflectance or magnification levels of the different OEC element types typically require different drive voltages, thus adding to the complexity of relatively accurate control of the OEC elements. Moreover, OEC elements come in a relatively large array of shapes and sizes which may require different drive voltages to compensate for voltage drops in the various coatings, solution, chemicals, and chemistry, for example, on the larger mirrors.
In order to provide the driver with acceptable glare levels from the IEC mirrors as well as the OEC mirrors, for example, during night driving, the drive voltages to each of the mirrors must be appropriately scaled. Since the IEC and the OEC elements do not share a common thermal environment, it has been relatively difficult if not impossible to correct for temperature-related performance changes in the OEC elements from the inside.
It is an object of the present invention to solve various known problems in the prior art.
It is yet another object of the present invention to provide a control system for OEC elements wherein the drive voltage for the OEC elements can be relatively accurately controlled.
Briefly, the present invention relates to a control system for a plurality of electrochromic mirrors, for example, used in automobiles, to control the glare level of the IEC elements used as rearview mirrors as well as the OEC elements used as sideview mirrors. The IEC element and each of the OEC elements are provided with an individual drive circuit. The drive circuits for the OEC elements may be customized to account for various factors, such as the type of curvature as well as the size and shape. Since individual drive circuitry is provided for the IEC element and each of the OEC elements, the reflectance of each of the elements can be relatively accurately controlled by way of glare signal generated inside the automobile. More particularly, the individual drive circuits for each of the outside mirrors can be used to scale the drive voltage for each electrochromic element to compensate for differences in the curvature or size as well as temperature of operation of the OEC elements. By providing individual drive circuits for each of the OEC elements, the need for two relatively heavy gauge conductors in order to limit the voltage drop and a ground referenced to the inside mirror and associated drive circuitry is eliminated, thus simplifying the manufacturing process. In particular, in the present invention, the ground voltage does not need to be referenced to the IEC element, thus only one conductor and chassis ground is sufficient. In one embodiment of the invention, the control system is adapted to control all the electrochromic elements to provide a relatively constant level of glare at a predetermined reference point, such as the driver""s eye level, from all of the electrochromic elements.